The present invention generally relates to the event detection and characterization in acquisition of waveform data and more particularly to a method of characterizing events in acquired waveform data from a metallic transmission cable using time domain reflectometry.
A time domain reflectometer (TDR) launches interrogating energy pulses into a transmission medium, such as shielded and unshielded twisted pairs, coaxial cables, and the like, at a given pulse repetition rate, depending upon the designated range of the TDR. During the periods between pulses, acquisition circuitry samples the cable to acquire data representative of reflections from flaws, discontinuities, or breaks in the cable. The reflections in the cable are timed from the time of transmission of the energy pulse to determine the range from the transmitter to such flaws, discontinuities, or breaks. Reflections may represent changes in wire gauge, splices, moisture in the cable, and the like. The acquired data is processed and displayed as a waveform trace on a display device, such as a cathode-ray-tube, a liquid crystal display, or the like.
A TDR notes any changes in the characteristic impedance of the cable under test. For a telecommunications copper facility or plant, the characteristic impedance is typically between 100 and 125 ohms. Most unshielded cables fall between 100 and 105 ohms. Shielded cable like T1 is typically about 125 ohms. Any change in the cable's impedance is displayed on the TDR display device as an upward bump, downward dip, or some combination of both deviating from a horizontal trace.
The TS100 and the TV110 Time Domain Reflectometers, manufactured and sold by Tektronix, Inc., Beaverton, Oreg. and assignee of the instant invention, are examples of TDRs for respectively characterizing telephone twisted pair cables and coaxial cable, such as in cable television systems and the like. The TV110 automatically selects the pulsewidth of the interrogating pulse and the gain of the receiver based on the initial span of the instrument. The acquisition hardware in the TV110 acquires a single data point for each launched interrogating pulse. Timing circuitry in the acquisition hardware varies the sample strobe timing in relation to the launched interrogating pulse to acquire data points at different time intervals to build a waveform data set over the selected transmission cable span. Increasing the span causes the instrument to select a new pulsewidth and gain and to reacquire new data samples over the new range. The acquired waveform data set is displayed on a liquid crystal display device and movable cursors are provided for locating the position of displayed faults.
U.S. Pat. No. 4,970,466 describes a multi-function test set for testing local area networks or LANs. A TDR function is described where a successive approximation routine is used to determine the signal level on the LAN cable. Predetermined offsets are added to the successive approximation results to set a baseline comparison amplitude for the TDR test. An interrogating pulse is launched into the cable at the same time a counter starts counting a 100 MHZ clock signal. Any return reflection from the cable that exceeds the baseline comparison amplitude stops the counter. The counter results times the clock rate in seconds times the velocity of propagation of the cable divided by two gives the approximate distance to the event. Repeated interrogations of the cable are made and averaged to improve the distance accuracy to the fault. While this reference detects the presence of major events in a cable, it does not calculate the height of an event nor does it calculate a return loss. Further, the base line comparison amplitude limits the ability of the instrument to locate small amplitude return reflections in the cable which could be impairing the cable quality. In addition, smaller faults farther down the cable after a major fault are not detected.
U.S. Pat. No. 5,128,619 describes a system and method for determining cable characteristics that uses a computer and a time domain reflectometer, such as the Tektronix 1503B, manufactured and sold by the assignee of the instant invention. The computer utilizes an application program to access data acquired by the TDR and calculates the length, attenuation, impedance, and existence of any bridge taps and other discontinuities on the selected pair of wires in a cable. One of the objectives of this invention is to detect significant discontinuities that will not allow the operation of digital telephones or digital communication on the cable. A significant discontinuity in this context is defined in the reference as an impedance change that is equal to or greater than twice the cable impedance or equal to or less than one half the cable impedance. The application program calculates the first, second, and third derivatives of the plurality of time samples of the acquired waveform data from the TDR. The width of the incident pulse is used to establish threshold values for clearing counters, finding a pulse edge, and zeroing all counters and flags. The software moves through the waveform data counting first derivatives or slopes of the data to determine valid pulse edges. If the positive or negative slope counter exceeds the pulse edge counter, a flag is set and the software determines if the peak amplitude of the event exceeds an amplitude threshold limit. This threshold limit is set to qualify significant events which would disqualify the cable from being used for digital communications. If the event amplitude exceeds the threshold, the second derivative is examined on each point from the pulse's peak location back toward the start of the pulse to determine where the second and third derivatives approximate zero. Using the slope of the second derivative zero crossing, the point where the slope has dropped to 18% of its value at the second derivative zero is found. Using the second derivative zero crossing point, the third derivative zero crossing point, and the 18% slope crossing point, the start of the pulse is calculated. While this method finds the starting point of the event, it is computationally expensive in that the first, second, and third derivatives of all of the waveform data points need to be calculated to determine the location of the event. Additionally, any noise on the waveform will have an adverse affect on the calculated derivatives. Further, this method does not calculate the return loss of the event.
U.S. Pat. No. 5,461,318 describes a time domain reflectometer that automatically determines certain characteristics of events in twisted pair cables. The TDR acquires and displays a trace of the acquired waveform data and automatically places cursors at the start of the incident pulse and at the start of a reflected pulse. However, the reference does not describe or hint at how the instrument determines the start of these events. Accurately determining the start of an event is necessary in making accurate distance measurements. The TDR further calculates the return loss of the event. This is accomplished in the reference by determining the minimum and maximum values for the incident pulse and the reflected pulse. The waveform in front of the respective pulses is scanned to determine minimum values for each. The maximum values for the pulses are determined by scanning to the right of the cursors. The differences between the respective minimum and maximum values are the respective amplitudes of the pulses. The return loss is calculated from the well known equation RL=20 log (incident pulse height/reflected pulse height). However, the reference does not teach or hint at how the minimum values of the event are calculated. Accurately determining pulse heights is important in accurately determining the return loss of an event. The distance to the event is determined by calculating the distance between the cursors and applying the velocity of propagation correction factor to the calculated distance. The distance to an event is only as accurate as the placement of the cursors at the start of the event and this has not been described in the reference. It can only be assumed that the cursors are manually placed by the user.
In the field of optical time domain reflectometry (OTDR), automated waveform acquisition and event detection and characterization has progressed farther than with metallic time domain reflectometers. U.S. Pat. No. 5,155,439, assigned to the assignee of the present invention, describes an optical fault locator that launches optical pulses into a test fiber at a first pulse width. The return reflected optical signal is converted to an electrical signal, digitized, stored and processed to locate anomalous events in the fiber. Any region of the test fiber having an anomalous event is reexamined using optical pulses having a pulse width optimized for that region of the fiber. A symbolic display is used to indicate the location and type of event located in the fiber instead of the traditional waveform trace.
U.S. Pat. No. 5,528,356, assigned to the assignee of the present invention, describes an OTDR that acquires and stores waveform data points having multiple waveform segments with each waveform segment having data points acquired using different pulsewidths, sample spacings and starting distances. Each waveform segment is defined in terms of the noise floor. The gain of the OTDR receiver amplifier may be increased for the various waveform segment acquisitions in conjunction with other parameters, such as the pulsewidth, averaging and the like, to increase the signal to noise ratio within the segment.
U.S. Pat. No. 5,365,328, assigned to the assignee of the present invention, describes a method for characterizing an event in acquired digital OTDR data where the event has a known shape, and a pattern having amplitude and location coefficients is applied to the data for determining a best fit between the data and the pattern as a function of a peak RMS value. This method is useful in characterizing nonreflective events in the OTDR data. However in metallic time domain reflectometry, it is very difficult to categorize the return reflective events to any particular shape without extensive computational effort. This would require a great deal of storage space for the algorithms performing the computations. In addition, the time required to perform the computations would far exceed the time a user would allow for receiving results from a cable examination.